Many products of commercial value (such as oil-) and water-repellant finishes for textiles, paper, electronic articles, and the like; stain-repellant finishes for leather; and surfactants for a variety of applications) can be made from perfluorinated carboxylic acid fluorides (hereinafter, perfluorinated acyl fluorides) and perfluorinated ketones. Perfluorinated acyl fluorides can be prepared by electrochemical fluorination (ECF) of the corresponding hydrocarbon carboxylic acid (or a derivative thereof), using either anhydrous hydrogen fluoride (Simons ECF) or KF.2 HF (Phillips ECF) as the electrolyte. However, a drawback of Simons ECF is that side reactions often occur, and low purity and low yields are often obtained due to the formation of rearrangement and degradation products. Perfluorinated acyl fluorides can also be prepared from telomers of tetrafluoroethylene, but a characteristic of this method is that a distribution of molecular weights is obtained.
Although Phillips ECF (KF.2 HF) or direct fluorination (F.sub.2) can be employed to reduce the occurrence of side reactions and provide high yields of a desired fluorinated product, hydrocarbon carboxylic acids cannot be fluorinated by such techniques without undergoing decarboxylation and/or other side reactions. Hydrocarbon carboxylic acid fluorides are extremely water-sensitive and difficult to handle, and hydrocarbon carboxylic acid chlorides yield chlorine-substituted fluorochemical products. In contrast, hydrocarbon carboxylic acid esters function well as starting compounds in both fluorination processes (see, e.g., U.S. Pat. Nos. 3,900,372 (Childs et al.) and 5,093,432 (Bierschenk et al.)), but, when hydrocarbon carboxylic acid esters are fluorinated, perfluorinated carboxylic acid esters (rather than perfluorinated acyl fluorides) are obtained. The perfluorinated esters are less useful than the corresponding acyl fluorides or ketones as intermediates in the preparation of commercial products, because perfluorinated esters, upon reaction with nucleophiles such as methanol, provide mixtures of products (derived from the acyl and the alkoxide portions of the perfluorinated ester) which may be difficult to separate. The side reactions of perfluorinated esters are especially limiting in making condensation polymers from difunctionals. Additional steps must therefore be undertaken to convert the perfluorinated esters to the more useful perfluorinated acyl fluorides and/or ketones.
U.S. Pat. No. 3,900,372 (Childs et al.) describes a combination process for the conversion of primary or secondary alkanols to perfluorinated acyl fluorides and/or perfluorinated ketones wherein said alkanols are esterified with acyl fluorides and the resulting partially fluorinated esters passed to an electrochemical fluorination step to produce perfluorinated esters which are thereafter cleaved on contacting with a source of fluoride ion under reacting conditions, e.g., a bed of a solid alkali metal fluoride catalyst at a temperature within the range of 80.degree. C. to 220.degree. C.
De Marco et al. disclose (in J. Org. Chem. 37(21), 3332 (1972)) that perfluorinated esters are decomposed in the presence of alkali metal fluorides at -78.degree. C. or above, but note that for higher molecular weight esters the rate of decomposition at -78.degree. C. is slow.